In the field of optics, a combiner is an optical apparatus that combines two images together, from either the same side of the combiner (reflective/reflective, or transmissive/transmissive) or from the two different sides of the combiner (reflective/transmissive). Often times, optical combiners are used in heads up displays (“HUDs”), sometimes referred to as head mounted displays (“HMDs”) or near-to-eye displays, which allow a user to view a computer generated image (“CGI”) superimposed over an external view. The HUD enables the user to view the CGI without having to look away from his usual viewpoint. The term HUD originated from its use in avionics, which enabled a pilot to view information while looking forward with his head up, as opposed to looking down at an instrument panel. Conventional HUDs include tilted dichroic plates, holographic combiners, angled transparent substrates, and compound conjugate lenses.
Two versions of combiners exist. The first version combines two fields without adding any lensing to either field (typically a tilted dichroic plate or compound conjugate lenses). The second version includes a lensing functionality, in addition to the combining functionality, which is usually an off-axis aspheric lensing for the field coming from the display. The lensing functionality is used to displace the virtual image originating from the display into the far field or at a specific distance from the combiner and to give the image a certain field of view to enable the user to bring the virtual image into focus at the target size.
Since near-to-eye displays by definition are worn close to the user's eyes, HUDs can physically interfere with conventional prescription corrective glasses. In some configurations, a HUD may be worn over prescription corrective glasses, but such configurations are often awkward and uncomfortable. Conventional near-to-eye displays are not well suited for use with users that require prescription corrective lenses, unless the user chooses to wear contact lenses. However, many people who need prescription corrective lenses choose not to wear contact lenses for a variety of reasons and are thus left with less than desirable options when wearing HUDs.